In recent years, titanium oxide particles have gained attention as a photocatalyst, and a method for producing the same has been examined. For example, Patent Documents 1 and 2 and Non-Patent Document 1 disclose titanium oxide particles (hereinafter, referred to as “decahedral titanium oxide particles”) formed in a decahedral box shape and mainly having an anatase-type crystal and a method for producing the same. It is described that the decahedral titanium oxide particles have a high activity as a photocatalyst since the decahedral titanium oxide particles have a large surface area per unit of the mass, a high crystallinity, and a small inner defect. In addition, in Non-Patent Document 2, it is described that the decahedral titanium oxide particles are largely expected as a photocatalyst since the ratio of a highly active (001) surface is high.
In the method of producing the decahedral titanium oxide particles disclosed in Non-Patent Document 2, since a hydrothermal reaction using hydrofluoric acid is used, the method is not suitable for industrial production. For this reason, we have additionally examined the method of producing the decahedral titanium oxide particles disclosed in Patent Documents 1 and 2 and Non-Patent Document 1.
In the method for producing the decahedral titanium oxide particles disclosed in Patent Documents 1 and 2 and Non-Patent Document 1, vapor of titanium tetrachloride and oxygen introduced into the reaction pipe are rapidly heated and cooled by heating the reaction pipe from the outside thereof, and the reaction expressed by the following chemical formula (1) is performed.TiCl4+O2→TiO2+2Cl2  (1)
According to the reaction shown in the chemical formula (1), a powder product containing decahedral titanium oxide particles was obtained in a region on the downstream side of the reaction region. However, the ratio of the powder product was less than 40% among the total product, and the rest was a white film-shaped product fixed to the inner wall surface of the reaction pipe in the reaction region.
As described above, in the method for producing the decahedral titanium oxide particles disclosed in Patent Documents 1 and 2 and Non-Patent Document 1, a large amount of the film-shaped product was fixed to the inner wall surface of the reaction pipe, which caused a problem that the productivity of the decahedral titanium oxide particles deteriorated.
In addition, when the reaction was continued in the state where the film-shaped product was fixed to the inner wall surface of the reaction pipe, a result was obtained such that the film-shaped product was more easily produced and the ratio of the decahedral titanium oxide particles among the powder product gradually decreased.
In addition, in the production method, the same result obtained in the case of producing the decahedral titanium oxide particles was obtained even in the case of synthesizing another titanium oxide particles. As a result, there was a problem that 50% or more of the product was the film-shaped product fixed to the inner wall surface of the reaction pipe.
For this reason, in order to efficiently and continuously produce the decahedral titanium oxide particles or the other titanium oxide particles, it is necessary to prevent the film-shaped product from being generated on the inner wall surface of the reaction pipe.